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Make me to understand the term efficacy in simple

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Efficacy, in simple terms, means how well a drug actually works to produce the effect you want, once it's acting on its target.
Think of it this way:
  • Potency = how little of the drug you need to get an effect (a "strong dose" concept).
  • Efficacy = the maximum effect the drug can ever produce, no matter how much you give.

Simple analogy

Imagine two painkillers:
  • Drug A needs only 10 mg to relieve your headache.
  • Drug B needs 400 mg to relieve the same headache.
Drug A is more potent (needs less to work), but if both drugs relieve the headache equally well at their respective doses, they have the same efficacy - they are equally effective treatments, just at different doses.

The technical definition

Efficacy is the maximum response a drug can produce when it binds to its receptor. It depends on:
  1. How many drug-receptor complexes form
  2. The drug's "intrinsic activity" - its actual ability to switch on the receptor once bound
This is why a full agonist (which fully activates the receptor) has high efficacy, while a partial agonist (which only partially activates it) has lower maximal efficacy - even if it binds the receptor just as tightly. An antagonist blocks the receptor but activates it not at all, so its efficacy is essentially zero.

Why efficacy matters more than potency clinically

A drug with higher efficacy is generally more clinically useful than one that's just more potent, because efficacy tells you the ceiling of benefit you can get - potency just tells you what dose gets you there. For example, ibuprofen and naproxen have the same maximum efficacy for pain/fever relief, just at different doses, so they're considered therapeutically equivalent for that purpose, even though naproxen is more potent (lower doses needed).
Source: Lippincott Illustrated Reviews - Pharmacology, p. 95-97
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